Radar level gauging in all kinds of tanks has found a steadily increasing use since the mid-seventies. The extension of the use to more diversified applications has posed a number of challenges on the design of radar level gauges.
Obvious basic functions for all radar level gauges are sealing of the tank, which may hold high pressure, dangerous contents etc., and sealing of the enclosure protecting the electronics of the radar level gauge, at least for outdoor environments applications and sometimes for open sea applications etc. The enclosure for the electronics is many times also determined by regulations for explosion protection related to electric equipment. This is all standard technology but a characteristic need for radar level gauges is an electrical (microwave) connection from the radar electronics to the antenna.
Coaxial connections and waveguides are used presently for this microwave connection. At least one microwave joint is usually provided in this microwave connection, for instance to be able to replace a faulty electronic unit while the tank is pressurized. The microwave joint is a critical part during field replacements.
Coaxial connectors are manufactured as standard items including sealed parts etc. and thus frequently used in radar level gauges. Field experience however is that the tiny connectors, which are necessary for high frequency use, may easily be damaged during handling outside of laboratory environments and that they will degrade severely if even a very small amount of water, a 0.1 mm layer or less, or dirt is trapped inside at handling during certain weather conditions. Waveguides are also frequently used and are much more robust but have to be well integrated in the mechanics in order to provide a cost efficient solution. Due to the larger dimensions of the waveguide the requirement for good electrical connection is less pronounced and local water droplets etc. can be tolerated. Normal waveguide impedance is a few 100 ohms instead of the 50 ohm impedance in a coaxial cable indicating much less current densities for the same transported power.
Sensitivity is a very critical property in applications where small antennas have to be used and where the low reflection of a turbulent surface of a low-epsilon liquid have to be taken care of. In a typical radar level gauge in a quiet refinery tank or corresponding tank on a ship in harbor, the two way attenuation for the radar signal may be 40 to 50 dB, while a typical process tank with a 2″–3″ antenna, turbulent surface etc. will exhibit a corresponding attenuation of 70 to 80 dB or more. Further, a coaxial cable for a 25 GHz radar will introduce a loss off several dB in a typical radar level gauge application, when used as a connection from the electronics to the antenna.
Furthermore all radar systems need some duplexing function to use the same physical antenna for transmitting and receiving and typically that introduces two-way losses of 6–10 dB or more with the simple system used in most radar level gauges. Power splitter or hybrid junction are examples of such systems both giving at least 3 dB in each direction. Two separate antennas would save these 6–10 dB but a more practical way is to use two antenna functions inside the same antenna mechanics. In that case right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) are sometimes used to form two independent antenna functions connected by separate lines to the microwave transmitter/receiver unit. A rotational symmetric horn antenna is one example of a physical antenna where these two electrically independent antenna functions LHCP/RHCP are easily housed.
Said RHCP/LHCP arrangement is also very useful for handling reflections to flat walls in the tank. Such reflections, both towards the surface and towards the walls, are known to change the circular polarization of the radar wave from RHCP to LHCP or the other way around, and by sending for instance RHCP and receiving LHCP only waves reflected an odd number of times (1, 3 etc.) will be received without extra attenuation. For many kinds of echoes, but especially for disturbing reflections in a flat wall within the antenna beam, this is known to be an efficient method to make the echo more clean and thus increase accuracy for a radar level gauge. In a big tank (used in refineries etc.), with mounting holes for big antennas available, a narrow antenna beam can be used (based on big antenna diameter) limiting the number of false reflections in tank structures but in a small tank, typical for many applications in the process industry, a wider antenna beam is necessary (due to small mounting holes for antennas) which combined with the more compact geometry will give a larger number of false reflections in tank structures.
In small tank applications there will always be internal reflections or bouncing back and fourth between the antenna and the microwave unit and such reflections will create a number of false echoes seemingly below the antenna but entirely created internally. Obviously such echoes are weak but nevertheless important at weak surface reflections due to turbulence, foam etc. or due to a dirty antenna surface.
One radar level gauge including the RHCP/LHCP-method is described in U.S. Pat. No. 5,543,720. The practical solution there is complex in order to cover a wide range of applications. The complex mechanics involved also indicates a far too high cost for the typical process use. Also, this arrangement is intended for use in connection with large storage tanks and not suited for measurements in smaller size tanks, such as process tanks, which usually are of substantially smaller physical dimensions.